Randy's comment made me search for neckers online, and I did find a knife that looks similar to my design. I guess I need to get out of the Busse Bubble more often because there are a lot of options I hadn't considered.

This morning I've read reviews on:

Bark River Bravo Necker
Becker Necker
ESEE Izula and Izula 2
Rat Hest

(NOTE: I own a Scrapivore and Warden)

They all have either enough handle or enough blade - but never both.

Apparently there is a glass ceiling in necker overall length wherein you can't have a full grip and 4" blade. It seems like most neckers are in the 6-6.75" range.

My proposed design is 8" long. This allows a full 4" grip and 4" blade with a 3-5/8" cutting edge (measured orthogonally - the actual linear dimension is greater).

ETA: The Yard could be the first (and best) producer of a truly USEFUL necker. Again, this could be the "one knife" you always have handing (hidden if necessary) around your neck when you really need a good knife.